The present invention relates, in general, to a method and apparatus for drilling generally horizontal boreholes, and more particularly to a guidance system for drilling such boreholes to a close tolerance to specified end points.
The technology for drilling boreholes into or through hills or mountains, under rivers and the like has been well developed over the years. However, unique problems arise when it becomes necessary to drill such a borehole in an area that is inaccessible, such as beneath a ship's channel in a river, or where multiple boreholes must be drilled in parallel to each other with a high degree of accuracy. In such situations, ordinary techniques for guiding the drilling of boreholes are not always satisfactory.
An example of the need for a high degree of accuracy in drilling boreholes is found in a recently developed procedure for boring horizontal tunnels in unstable Earth. This procedure requires drilling a number of parallel boreholes of small diameter with a high degree of accuracy around the circumference of the tunnel. The boreholes may be, for example, six inches in diameter, with about 40 boreholes positioned around the circumference of the tunnel to form a circle about 20 meters in diameter. The holes are drilled into the hill or mountain in which the tunnel is to be excavated, and are cased with plastic pipe. A refrigerant is then pumped through the casings for an extended period; for example, one month, to freeze the soil. Thereafter, the Earth inside the circle formed by the boreholes is excavated using conventional techniques to produce a tunnel in which the tunnel wall is supported by the frozen Earth. The tunnel may extend partially into the hill or completely through it.
A major problem with the foregoing technique is how to drill a large number of parallel boreholes around the circumference of a tunnel while keeping the boreholes accurately spaced and parallel to each other so as to properly define the tunnel.
Another example of the need for accurate drilling of generally horizontal boreholes is that of drilling boreholes under an obstacle such as a river, where the surface of the Earth above the borehole is not accessible for conventional surface guidance techniques. Such a situation can occur when a borehole is to be drilled under a river to exit at a specified location, but where the river includes an inaccessible region such as a ship's channel. Such a borehole may be started on the near side of the obstacle, with the object of drilling under it to a specific exit point on the far side. Conventional directional drilling techniques can be used to guide the drill at its entry and can provide general control for a portion of the distance. However, such control techniques have limited accuracy, so that a number of boreholes may have to be drilled before the desired exit point is reached.
The prior art describes the use of grids on the surface of the Earth to guide borehole drilling, but if access to the surface above the borehole is not available, this technique cannot be used effectively. Thus, for example, such grids may be placed on the Earth's surface at the banks of a river to provide drilling guidance. However, these grids have a limited range and may not be effective if the borehole is off target when it reaches the grid, for there may not be enough distance to allow the borehole to be turned to reach the exit point.
Thus, there is a need to provide a simple, easy-to-use, effective and accurate method and system for guidance of boreholes, and more particularly to guidance of the drilling of boreholes parallel to a predetermined linear path within small tolerances.